1. Field of the Invention
The present invention relates to a device for displaying letters and images, and more particularly, relates to various display devices using a polymer dispersed liquid crystal panel.
2. Related Background Art
Liquid crystal display devices of various kinds have been investigated as flat panel display panels up to now. The panel size varies depending on the applied field, for example, a large panel is used for a work station or a personal computer, a panel with a diagonal line of several inches is utilized for a television or a projection television, and a panel with a diagonal line of approximately one inch is used for a view finder or a head mounted display. Corresponding to the decrease in panel size, the pixel size becomes smaller, which provides difficulty in achieving a high-resolution panel. In the case of a liquid crystal display panel of the transmissive type, the aperture ratio of a liquid crystal panel cannot be increased because of the matrix electrode pattern, the orientation defect of liquid crystal, or orientation failure caused by a lateral electric field. For example, the aperture ratio is approximately 30% for a panel which has a diagonal line of one inch and has 100,000 pixels. When a pair of polarizers or a color filter is applied thereto, the transmissivity becomes as small as approximately 3%, which also impedes achievement of a high-resolution panel.
To solve the above mentioned problems, a reflective type liquid crystal panel has been investigated, which is expected to have an aperture ratio of more than twice as large as that of transmissive type liquid crystal panel.
The total transmissivity is, however, still too small under present conditions because of the low transmissivity of the polarizing plate and color filters, even though the aperture ratio is improved by the reflective type liquid crystal panel. Since absorption or diffusion color filter including pigments has low color selectivity, the transmissivity should be usually decreased to around 40% so as to provide enough color cross-talk. Moreover, in the case of a reflective type liquid crystal panel, as the light has to be transmitted through color filters twice, it will lose the portion which is kept from losing by the aperture ratio improvement. Thus, the transmissivity becomes low when a high-resolution panel is requested for a liquid crystal display device. To cover this problem, a large high-intensity light source consuming a huge amount of electric power and a large-scale illuminating optical system comprising a big collective mirror for illuminating the panel and a cold filter are required. In other words, there are such problems as the display device becomes larger and heavier, a huge amount of electric power is required, the light utilization efficiency and production rate become low, and production cost is higher.